"Penyun" is a generally derogatory term for players of the Club Penguin game.
The word originated from The Old Forums when forum regulars grew tiresome of the younger and less Internet-savvy gamers. "Penyun" is a intentional misspelling of the term "penguin", a purposeful corruption to poke fun at Club Penguin players. Recently it's developed as a lolspeak version of the word "penguin" as well, losing some of it's negative connotations.
This term was usually used to describe Club Penguin players as a whole; however while "penguin" was used for all players, "penyun" was used only for the ones who bothered or annoyed other posters. When Club Penguin split to its new forum, the word quickly fell out of usage and today is mostly used when long-time posters from the old forums reminisce about years gone by.
Since its origin on The Old Forums the term has since spread to other Club Penguin communities, still used as a derogatory term for Club Penguin players. The usage mostly remaining the same, it's now sometimes also used to jokingly refer to Club Penguin players in a non-derogatory way, for the sake of playfulness.